Mechanisms of ventral body wall closure

The abdomen (belly) of a baby is formed in the womb by the wrapping of the internal organs, first by a thin layer of so-called mesoderm and ectoderm ("primary body wall") and then by muscle and connective tissue ("secondary body wall"). How the secondary wall replaces the primary is not understood, but a snug fit is maintained throughout the process because the internal organs are growing rapidly at the same time. About one in 2000 babies are born with a defect in the abdominal wall, which most often presents clinically as a hole in the abdomen through which parts of the gut and sometimes the liver protrude (herniate). The causes of abdominal wall defects are also not well understood, in part because the basic embryology involved has not been studied in a detailed quantitative way. In the proposed project, we will use the mouse embryo, in which abdominal wall closure is very similar to that in humans, as a model to understand the details of growth, reshaping and physical forces that must work together to enable the secondary body wall to form. We will first make a 'growth atlas' of the abdominal wall to determine how the entire process coordinates replacement of the primary wall by the secondary wall while containing the internal organs. To link the anatomical changes with our understanding of cell biology, we will look in fixed specimens at the cells that make up all the tissues to see how the growth and re-shaping are achieved by cell multiplication, rearrangement, and size change, and take pieces of living embryonic tissue in which cells are fluorescently labelled so that we can microscopically observe these cell behaviours individually. We will compare embryos from healthy mice with those from mouse mutants known to have frequent abdominal closure defects to see which cellular processes are abnormal so as to cause abdominal defects. Finally, we will test the idea that the abdominal wall growth is triggered and coordinated by mechanical tension, exerted by primary body wall narrowing and pulling on the secondary body wall and by expansion of the growing internal organs. To do this, we will map tension across the growing abdominal wall and see if it correlates in space or time with cell proliferation and make small incisions to relieve local tension, investigating whether this reduces local proliferation. We will also investigate a molecular signalling pathway, known as Yap/Taz, that is known in other situations to stimulate cell proliferation in response to tension, and ask if it is present, active, and necessary for abdominal wall growth. Together the investigations in this project will provide an integrated understanding of abdominal wall closure, establishing the basic biology and elucidating ways in which birth defects might occur.

Failure of the abdominal wall to form properly is a serious birth defect but what exactly fails is not understood. Abdominal wall formation requires flank mesoderm (secondary body wall, SBW) to extend ventrally around the body, progressively replacing the primary body wall (PBW), a thin layer of embryonic mesoderm and ectoderm that wraps around the viscera as the embryo grows out from disc/cup stages. This process, ventral closure, is known as "ventral migration", but it is unknown how much cell migration is involved versus bulk SBW growth and PBW narrowing. Also unknown is how the PBW narrows, although models of omphalocoele (postnatal gut herniation) include mouse mutants in polarity genes Celsr1 and Scribble, associated in other tissues with convergent extension, suggesting a mediolateral cell intercalation mechanism. Coordination of SBW and PBW with visceral growth is also mysterious: the gut normally herniates prenatally into the umbilicus for a period just before final closure, suggesting just-sufficient closing force. To elucidate ventral closure we will deploy our established techniques to (1) quantify shape and volume changes in PBW, SBW and viscera, and measure proliferation, orientated division, rearrangement/migration and cell size/shape in wild type and mutants to establish the timing, localisation and cellular mechanisms of closure-associated directional growth at a cell-biological level; (2) determine, by live imaging of explants, the role of mediolateral cell intercalation in the PBW and ectoderm and the effect of Celsr1 and Scribble mutations on this in all ventral tissues; and (3) determine when and where PBW, SBW and skin are under mechanical tension, the effect on proliferation of that tension, and the role of the Yap/Taz tension-sensing pathway in coordinating PBW and SBW morphogenesis with visceral growth. Together these investigations will provide an integrated understanding of ventral closure, a crucial developmental process.